I. Field of the Invention
The present invention relates generally to multi-sheet structures and methods for manufacturing such multi-sheet structures.
II. Description of Related Art
There have been previously known multi-sheet metal structures in which a first and second metal sheet are positioned together so that the second sheet overlies at least a portion of the first sheet. A blowing agent is contained with one or more selected potions between the first and second sheet and the first and second sheet are then joined together to form a continuous joint which encloses the selected portion containing the blowing agent. Joining techniques include welding, diffusion bonding, adhesive bonding and the like. Thereafter, the now attached first and second sheets are heated to a temperature which exceeds the decomposition temperature of the blowing agent so that the first and second sheets expand outwardly from each other in the selected portion containing the blowing agent. Oftentimes, the first and second sheets are positioned within a mold during the heating step so that the mold defines the final shape of the structure.
These previously known multi-sheet metal structures, however, have suffered from certain limitations such as need for dedicated surface pretreatments including removal of contamination and surface oxide are required prior to joining metal sheets. In addition, sheet surfaces to be welded must fit up accurately prior to joining. Welds made by other welding techniques (such as laser welding and electron beam welding) typically suffer from the fact that due to the melting of the base material and high cooling rates, the weld becomes brittle and weak and is also prone to stress concentration. This change in metallurgical property significantly affects the ability of the sheet to be formed when the blowing agent decomposes. Some prior joining techniques (such as diffusion bonding) require lengthy processing time to join the multiple sheets as opposed to friction stir welding which requires only a few minutes. Furthermore, it is imperative that during the thermal welding step that the blowing agent not decompose until the thermal weld is completed. Consequently, in order to avoid premature decomposition of the blowing agent during the welding operation, it has been previously necessary to form a weld at a relatively large distance away from the blowing agent or otherwise take steps to ensure that the blowing agent remains below its decomposition temperature during the welding operation. This, in turn, limits the type of structures that may be manufactured.
A still further disadvantage to the previously known methods for fabricating multi-sheet metal structures which utilize blowing agents, is that it is difficult to fabricate structures having three or even more sheets in which the weld pattern between the first and second sheet differs from the weld pattern between the second and third sheet. This difficulty arises primarily since it is difficult to control the depth of the thermal welding between only two of the three sheets.